The present invention relates to optical networking, and more particularly in certain embodiments to systems and methods for unidirectional broadcast of information such as, e.g., video information.
High-performance optical networks have facilitated the emergence of the Internet as a widely prevalent medium for business and personal communications. Principal applications enabled by such networks include, e.g., large-scale web access, voice over Internet telephony, electronic mail, peer-to-peer file sharing, etc. A common attribute of these particular applications is that they involve bidirectional communication of data.
FIG. 1 depicts a representative node 102 in a typical bidirectional optical network architecture. Multiple wavelengths λ1, λ2, etc. enter and leave node 102 via optical fiber in two different directions. The information received on a particular wavelength λxx. present on light flow in a first direction is to be made available for a local client 104. Information generated by client 104 is modulated onto the same λxx on light flow in the opposite direction so as to return to the source of the received data. Client 104 may itself represent communication with another network such as networks associated with a particular locality, region, building, etc.
The wavelength λxx is separated from a multi-wavelength signal input to node 102 (arriving from the left in FIG. 1) by a demultiplexer 106. Alternatively, the drop portion of an optical add-drop multiplexer may substitute for demultiplexer 106. A locally generated λxx is inserted into the light flow output of node 102 to the left by a multiplexer 108, which may be replaced by the add portion of an optical add-drop multiplexer.
Client 104 will typically not be configured to participate directly in the multi-wavelength optical network. If client 104 communicates via an optical interface, the wavelength or wavelengths used will only be locally specified and will not be selected in accordance with a wavelength scheme globally defined for the optical network. Also, any optical signals transmitted by client 104 will not necessarily be amplified to the level required for inter-node transmission in the network. The optical signals transmitted and received by client 104 will be modulated with client data. Alternatively, client 104 will have an electrical interface. A bidirectional transponder 110 will thus play a key role in this architecture.
Bidirectional transponder 110 amplifies and demodulates wavelength λ xx, recovers the data carried by λxx, and then uses this data to re-modulate an optical or electrical signal to client 104. In the other direction, transponder 110 receives an optical or electrical signal from client 104, recovers the data and uses it to modulate an optical signal on wavelength λxx and amplify this optical signal to a launch power appropriate for transmission to the node originating the incoming λxx signal. The wavelength λxx is specified within a scheme defined for the optical network as a whole. One example of a commercially available implementation of transponder 110 is the ONS15454 10G MR Transponder available from Cisco Systems of San Jose, Calif.
An emerging application for the Internet and high-speed optical networks is video distribution. This is a unidirectional application. The remote clients do not generate data. Each node is responsible for recovering data from the wavelength dedicated to video distribution for client processing and for retransmitting that information on to the next node.
It would be desirable to use bidirectional transponders as currently exist for this application. This is because such transponders are already readily available and also because it is desirable to combine unidirectional and bidirectional applications in the same network. For example, it may be desirable to dedicate certain wavelengths to bidirectional applications and other wavelengths to unidirectional applications without requiring separate unidirectional and bidirectional transponders.
Problems arise, however, in attempting to utilize the presently available bidirectional transponders in unidirectional applications. Bidirectional transponders do not inherently include any capability for relaying information on one or more selected wavelengths on to a next node. This is because there is no internal coupling between the network side input and the network side output.
FIG. 2 depicts a prior art approach to employing bidirectional transponders for digital video distribution. A client 202 outputs a single-wavelength optical signal modulated with broadcast video information. This optical signal is presented to a bidirectional transponder 204. Bidirectional transponder 204 recovers data from the client signal and re-modulates it onto a wavelength selected for use in a multi-wavelength optical network. The network side output of transponder 204 is an input to a multiplexer 206 or the add portion of an optical add-drop multiplexer.
A representative node 208 is responsible for both recovering the video signal for its own client and for relaying the wavelengths carrying the video information on to the next node. Node 208 includes a splitter 210, which taps off a portion of the available optical power to be sent to a demultiplexer 212. Demultiplexer 212 selects the particular wavelength being used for unidirectional traffic and presents it to a bidirectional transponder 214. Bidirectional transponder 214 then recovers the video data from the unidirectional wavelength and re-modulates it onto an optical or electrical signal to be presented to a client 216. The bidirectional transponders 204 and 214 are made to work in the unidirectional application by simply omitting the client-side input for the selected unidirectional wavelength. Optical signal flow to the next node is optically amplified by amplifier 218.
The reliance on optical splitting and optical amplification for forwarding the unidirectional channels to the next node brings certain drawbacks. The insertion loss of splitter 210 imposes a performance loss, which may reduce the maximum spacing between nodes. Also, as the optical signal traverses the network, there is a limit to how many stages of purely optical amplification may be used before optical-electrical-optical conversion and regeneration will be necessary. Yet the capabilities of the bidirectional transponders in this respect are left unused. Furthermore, the architecture of FIG. 2 is not applicable to requirements for combined unidirectional and bidirectional operation.
What is needed are improved systems and methods for providing unidirectional communications via optical networks that employ bidirectional transponders.